1. Field of the Invention
The present invention relates to a magnetic recording apparatus and a signal processor for use with it.
2. Description of Related Art
The magnetic recording apparatus moves its head radially in relation to a rotating disk to accurately position the head over a target data track and magnetically performs a write or read operation. FIG. 2 shows a top view of the inside of a typical enclosure of the magnetic recording apparatus 1. FIG. 3 is a cross-sectional view of the magnetic recording apparatus. As an example, FIG. 3 shows the magnetic recording apparatus 1, which mainly consists of six heads 2, three disks 3, a rotary actuator 4, a voice coil motor 5, a head amplifier 6, and a package board. The three disks 3 are fastened to a rotation shaft, and rotated around point A when driven by a spindle motor at a speed of 3,000 to 15,000 revolutions per minute. The six heads 2 are fastened to a comb-shaped arm, and rotated around point B when driven by the rotary actuator 4. Thanks to this mechanism, the heads 2 can freely move in the radial direction over the disks 3. Since the rotary actuator 4 is suitable for mechanism downsizing, the actuators employed for recently released magnetic recording apparatuses are of the same type as the rotary actuator 4. Further, the disks 3 have servo areas 7, which are provided at nearly equal angular intervals, to detect the radial positions of the heads 2. The locations of the servo areas 7 and data areas 8 and the means for detecting the radial positions of the heads 2 from the servo areas 7 will be detailed later. The package board carries a hard disk controller (HDC), interface circuit, signal processing unit, and other components for control purposes. The head amplifier 6 is often mounted within the enclosure and near the heads 2 for the purpose of enhancing the S/N ratio and transfer rate.
FIG. 4 is an enlarged detail top view of part of a disk 3. The rotary actuator 4 can position the head at any radial position of disk 3. However, the head is fixed at a specific radial position when data is to be read or written. As indicated in FIG. 4, concentric tracks 9 are formed at nearly equal spacing intervals. In the figure, only five tracks 9 are indicated by solid lines for explanation purposes. In reality, however, the tracks 9 are magnetically formed and cannot directly be viewed by any optical means. Further, the figure shows the tracks with their widths magnified. In the actual magnetic recording apparatus 1, however, a total of several tens of thousands of tracks are formed over the entire surface of disks 3 and positioned at spacing intervals of smaller than 1 μm.
Thanks to the technology disclosed by JP-A NO. 222468/1983, special patterns called servo patterns are factory stored on the disks prior to product shipment and widely used to acquire a head position signal for the purpose of following a specific track. The servo patterns are formulated in the servo areas indicated in FIG. 4 and FIG. 5. The servo areas and data areas are isolated from each other by gap sections, which are provided to absorb rotation speed variations. Each data block is divided into sector blocks. Each sector block consists of about 600 bytes and has management information in addition to 512-byte user data. The data areas greatly differ from the servo areas in that the data areas are frequently rewritten by a command from the user, whereas the servo areas are not rewritten after product shipment. About 50 to 100 servo areas are formed on the disk surface and positioned at nearly equal angular intervals. Since the number of data areas is larger than that of the servo areas, there are several data areas between certain servo areas. In reality, the magnetic recording apparatus has more than 10,000 tracks. However, the contents of FIG. 6 are enlarged significantly in the vertical direction. The servo area is a pattern in which radially adjacent tracks are time-synchronized in the bit direction. For the formation of such a special pattern, a clock synchronized with disk rotation is needed. The servo track writer, which has such a special pattern formation function, is used in conjunction with the technology disclosed by JP-A No. 48276/1989 to formulate the servo areas.
Within the pattern illustration in FIG. 6, the ISG (Initial Signal Gain) block is a continuous pattern that is provided to reduce the influence of disk recording film magnetic characteristics and levitation irregularities. The servo demodulator circuit turns ON the auto-gain control (AGC) to reproduce the ISG block. When the SVAM (Servo Address Mark) block is detected, the AGC is turned OFF to standardize the subsequent reproduced amplitude of the burst block with the ISG block amplitude. Further, when the SVAM is detected, the reproduction system switches to the servo mode for the detection of a gray code, servo sync, and the like. The gray code block is a place where the track number information about each track is written in gray code. In this block, the sector number information may also be written. The burst block is a houndstooth check pattern for acquiring the accurate information about radial position. It is necessary for accurate track following by the head. This pattern consists of a set of A and B bursts and a set of C and D bursts. The A and B bursts equally extend over the center of each track. The C and D bursts equally extend over the centers of adjacent tracks. The pad block is a pattern for absorbing the delay in the demodulator circuitry in order to assure continued clock generation for the period of servo area reproduction by the servo demodulator circuit.
As shown in FIG. 7, the gray code block uses a set of two magnetization changes (dibit) to represent information, achieves equalization with a filter similar in characteristics to a dibit waveform matched filter, and effects demodulation according to a signal amplitude level judgment.
The longitudinal magnetic recording method does not respond to DC magnetization and generates a single-peaked output in relation to magnetization changes only. Therefore, the reproduced waveform derived from recorded magnetization shown in FIG. 8A looks like FIG. 8B. When double-layer perpendicular media having a soft magnetic under layer are used, the resulting reproduced waveform contains a DC component as shown in FIG. 8C. Since the actual reproduction circuitry shuts out the DC component, the resulting reproduced waveform is distorted. Therefore, when integrating detection is attempted, the correct position signal will not be obtained due to the influence of the DC offset. Even when integrating detection is not attempted, the dynamic range of the analog-to-digital converter (ADC) needs to be provided with a margin for distortion. Therefore, inadequate quantization accuracy results. As indicated in FIG. 7, the reproduced waveform in the gray code block is obviously different from a situation where longitudinal recording media are used. It means that gray code demodulation cannot be achieved by conventional LSIs.
Further, the burst signal in the gray code block or burst block is surrounded by a large DC erasure block as shown in FIG. 6. The shorter the wavelength, the smaller the demagnetizing field strength within recording bits. Therefore, the double-layer perpendicular recording method is characterized by the fact that the degree of thermal demagnetization increases with an increase in the wavelength. FIG. 9 deals with typical simulation results and shows the chronological changes in the reproduced output at various recording densities. The figure indicates that the degree of output reduction increases with a decrease in the recording density or an increase in the bit wavelength. Owing to the same effects, the magnetic field generated due to aforementioned DC erasure block magnetization affects the neighboring servo signal block and promotes thermal demagnetization of the servo signal block.
It is also reported that a shift in the direction of the track width occurs during bit recording depending on whether the edge of a recording bit track agrees with the polarity of prerecorded DC magnetization in situations where DC magnetization is prerecorded. In the servo area, therefore, an edge shift also occurs in like manner, degrading the position signal quality.
Further, the maximum bit length designed for the conventional servo area is greater than the maximum bit length of the data area. In such a state, the servo area exhibits the lowest thermal demagnetization resistance and cannot provide assured reliability.